The 1983 birth cohort of the National Linked Birth/Infant Death Data Set was analyzed to document the contribution of live born infants weighing less than 750 grams to the U.S. infant mortality rate. Previously, linked birth and death certificates for the total U.S. were not available, preventing assessment of the mortality experience of these least viable infants. This information is necessary in order to target interventions to reduce the infant mortality rate. The data set represents an intensive effort by NCHS to obtain data from all states and reporting areas. The analysis has been completed and accepted for publication by the American Journal of Public Health.